Interlock circuit

ABSTRACT

An interlock circuit comprises a plurality of semiconductor switching units electrically connected or connectable in series in a supply line, a plurality of control units electrically connected to the plurality of semiconductor switching units, and a trigger unit electrically connected to the plurality of control circuits. Each control unit activates or deactivates a corresponding semiconductor switching unit in response to trigger signal received from the trigger unit.

BACKGROUND

Printing devices often comprise a safety interlock system adapted to deactivate or switch off moving parts in case of risk or danger. For instance, the interlock system may be triggered by a sensor, such as when a user inadvertently opens a cover of the paper feeding mechanism of the printing device during the printing process. The interlock system then instantly stops the paper feeding mechanism so as to prevent harm or injury to the user.

Some safety interlock systems rely on mechanical switching relays. Mechanical switching relays are known to be reliable, but have limited lifespans, can generate current spikes when connecting or disconnecting capacitive or inductive loads, and are ultimately expensive to fabricate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an example interlock circuit.

FIG. 2 is a schematic circuit diagram of another example interlock circuit.

FIG. 3 is a schematic circuit diagram of an example interlock circuit with a diagnostics unit.

FIG. 4 is a schematic circuit diagram of another example interlock circuit with a diagnostics unit.

FIG. 5 is a flow diagram illustrating an example interlock method.

DETAILED DESCRIPTION

Specific examples of an interlock circuit and method according to the present disclosure will now be described in further detail with reference to the accompanying drawings.

FIG. 1 is a schematic illustration of an interlock circuit 10 according to an example of the present disclosure. The interlock circuit 10 comprises two semiconductor transistor elements 12 a, 12 b connected in series in a power supply line 14 of an electrical appliance (not shown). The electrical appliance may be a paper feeding mechanism of a printing device. However, this is just an example, and the interlock circuit 10 may be employed to provide a safety interlock in a large variety of electrical appliances or machinery.

The power supply line 14 may form part of the electric circuitry or control circuitry of said electrical appliance, and may for instance provide electric power to said paper feeding mechanism of said printing device.

As can be further taken from FIG. 1, the interlock circuit 10 comprises a first control unit 16 a electrically connected to a gate electrode of the first semiconductor transistor element 12 a, and a second control unit 16 b electrically connected to a gate electrode of the second semiconductor transistor element 12 b. The first control unit 16 a and first semiconductor transistor element 12 a establish a first interlock path, and the second control unit 16 b and second semiconductor 12 b establish a second interlock path. The first and second interlock path can be connected in parallel in said interlock circuit 10.

In further detail, both the first control unit 16 a and the second control unit 16 b can be connected in parallel via a trigger line 18 to a trigger unit 20 comprising three switch elements 22 a, 22 b, 22 c connected in series. For instance, each of the switch elements 22 a, 22 b, 22 c may be associated with a corresponding cover or lid of a paper feeding mechanism of the printing device. FIG. 1 shows a trigger unit 20 with three switch elements 22 a, 22 b, 22 c. However, this is a mere example, and in general the trigger unit 20 may comprise a smaller or larger number of switch elements.

In normal operation of the printing device, all the lids and covers of the printing device are closed, which may correspond to each of the switch elements 22 a, 22 b, 22 c being in a closed state. In case all the switch elements 22 a, 22 b, 22 c are closed, the first control unit 16 a and the second control unit 16 b may apply respective control signals to the first semiconductor transistor element 12 a and the second semiconductor transistor element 12 b to close the semiconductor transistor elements, thereby enabling a current flow in the supply line 14. The printing device may hence be activated.

If, however, a user on purpose or accidentally opens a lid or cover of the paper feeding mechanism of the printing device, the corresponding switch element 22 a, 22 b, or 22 c will open, leading to an interruption of the current in the trigger line 18. This interruption constitutes a trigger signal provided from the trigger unit 20 via the trigger line 18 to the first control unit 16 a and second control unit 16 b. In response to the trigger signal, the first control unit 16 a deactivates the first corresponding semiconductor transistor element 12 a, and the second control unit 16 b deactivates the corresponding second semiconductor transistor element 12 b, thereby interrupting the supply line 14 at the positions of the first and second semiconductor transistor elements 12 a, 12 b. The interruption of the supply line 14 may instantly stop the paper feeding mechanism of a printing device, and will hence prevent harm or injury that could occur if the user came into physical contact with the moving parts of the paper feeding mechanism.

As can be taken from FIG. 1, each of the first control unit 16 a and second control unit 16 b independently deactivates the corresponding first semiconductor transistor element 12 a and second semiconductor transistor element 12 b, respectively. Since the first semiconductor transistor element 12 a and the second semiconductor transistor element 12 b can be connected in series in the supply line 14, a deactivation of either the first semiconductor transistor element 12 a or the second semiconductor transistor element 12 b would be sufficient to interrupt and deactivate the electrical appliance. However, the presence of two independent interlock paths via the first control unit 16 a and the first semiconductor transistor element 12 a on the one hand, and the second control unit 16 b and the second semiconductor transistor element 12 b on the other hand provides redundancy and can help ensure a reliable safety interlock even in situations in which either the first semiconductor transistor element 12 a and/or the first control unit 16 a, or the second semiconductor transistor element 12 b and/or the second control unit 16 b should fail.

Hence, by providing the first and second semiconductor transistor elements 12 a, 12 b and the first and second control units 16 a, 16 b redundantly, the reliability of the interlock circuit 10 can be significantly enhanced. For instance, assuming that a failure occurs with a probability p in the first interlock path comprising the first semiconductor transistor element 12 a and the first control unit 16 a or in the second interlock path comprising the second semiconductor transistor element 12 b and the second control unit 16 b, and assuming that failures affect the first control path and the second control path independently, the probability of a failure of the entire interlock circuit 10 is reduced from p to p².

A spatial separation between the first semiconductor transistor element 12 a and the second semiconductor transistor element 12 b may reduce the likelihood that failures affect both the first and second semiconductor transistor elements 12 a, 12 b at the same time. For instance, the failure tolerance may be significantly enhanced by ensuring a minimum spatial separation of 2.5 mm between the first semiconductor transistor element 12 a and the second semiconductor transistor element 12 b.

Similarly, a minimum spatial separation of 2.5 mm between the first control unit 16 a and the second control unit 16 b may prevent failures or malfunctions that simultaneously affect both the first and second control units 16 a, 16 b.

FIG. 1 shows a configuration of an interlock circuit 10 with two semiconductor transistor elements 12 a, 12 b and two control units 16 a, 16 b. However, this is a mere example, and in general the interlock circuit 10 may comprise a larger number of semiconductor transistor elements arranged in series in the supply line 14, and a corresponding plurality of control units, thereby establishing a larger number of independent interlock paths. Given that one single interlock path is sufficient to interrupt the supply line 14, the reliability and failure tolerance of the interlock circuit 10 can be further enhanced by increasing the number of independent interlock paths.

FIG. 2 is a schematic circuit diagram which illustrates the components of the example interlock circuit 10 of FIG. 1 in additional detail. Corresponding components in the circuit diagrams of FIGS. 1 and 2 share the same reference numerals.

As can be taken from FIG. 2, the first control unit 16 a comprises a first semiconductor transistor 24 a connected to the gate electrode of said first semiconductor transistor element 12 a. A gate electrode of said first semiconductor transistor 24 a is in turn connected to the trigger line 18 via two first resistor elements 26 a, 26 a′. The first control unit 16 a further comprises a second semiconductor transistor 28 a connected in series with said first semiconductor transistor 24 a of said first control unit 16 a to said gate electrode of said first semiconductor transistor element 12 a. Second resistor elements 30 a, 30 a′ can be connected in series between a gate electrode of said second semiconductor transistor 28 a and said trigger line 18.

Hence, as can be taken from FIG. 2, the first control unit 16 a comprises two independent control paths for the trigger signal, a first control path comprising the first resistor elements 26 a, 26 a′ and the first semiconductor transistor 24 a, and a second control path comprising the second resistor elements 30 a, 30 a′ and the second semiconductor transistor 28 a.

The first control circuit 16 a hence provides alternative and redundant signal paths across the first resistor elements 26 a′, 26 a and first semiconductor transistor 24 a, or across the second resistor elements 30 a′, 30 a and the second semiconductor transistor 28 a to control the first semiconductor transistor element 12 a. This allows to provide redundancy, and ensures a reliable interlock even in situations in which one or several of the elements 24 a, 26 a, 26 a′ in the first control path, or one or several of the elements 28 a, 30 a, 30 a′ in the second control path should fail.

The second control unit 16 b connected to the gate electrode of the second semiconductor transistor element 12 b generally corresponds in design and functionality to the first control unit 16 a, and comprises a first control path with a first semiconductor transistor 24 b and two first transistor elements 26 b, 26 b′ connected in series, and a second control path comprising a second semiconductor transistor element 28 b and two second resistor elements 30 b, 30 b′ connected in series. The first control path comprising the elements 24 b, 26 b and 26 b′, and the second control path comprising the elements 28 b, 30 b, 30 b′ can be connected in parallel so to provide redundancy in case one or several of the elements and either the first control path or the second control path of the second control unit 16 b should fail.

As can be further taken from the circuit diagram of FIG. 2, the first and second control units 16 a, 16 b can be supplied by the input voltage VIN of the supply line 14 via respective first and second control unit supply lines 32 a, 32 b, respectively. Each of the first control unit supply line 32 a and the second control unit supply line 32 b comprises a plurality of resistor elements. In the circuit diagram of FIG. 2, the input supply voltage VIN also supplies the trigger unit 20, whereas VOUT denotes the output voltage of the supply line 14 at the output of the second semiconductor transistor element 12 b.

FIG. 3 is a circuit diagram of an example interlock circuit 10 that largely corresponds to the interlock circuit described above with reference to FIGS. 1 and 2, respectively, and the same reference numerals are used to designate like components.

However, in addition the interlock circuit 10 of FIG. 3 comprises a diagnostics unit 34 adapted to test the functionality of the first and second semiconductor transistor elements 12 a, 12 b and the first and second control units 16 a, 16 b.

As can be taken from FIG. 3, the diagnostics unit 34 is powered by means of the input voltage VIN, and is electrically connected to the supply line 14 between the first semiconductor transistor element 12 a and the second semiconductor transistor element 12 b. At this electrical connection, the diagnostics unit 34 applies a voltage VOUT_TEST, thereby serving as a polarization circuit for the first and second semiconductor transistor elements 12 a, 12 b. The trigger unit 20 may now be activated to provide a trigger signal and deactivate the first and second semiconductor transistor elements 12 a, 12 b. An output voltage VOUT_TEST_SENSE may be sensed at an output node of the diagnostics unit 34, and may allow to infer malfunctions of the first and second semiconductor transistor elements 12 a, 12 b. In particular, comparison of the output test voltage VOUT_TEST_SENSE against pre-determined reference voltages allows to detect potential failures in the first and second semiconductor transistor elements 12 a, 12 b, or in the respective first and second control units 16 a, 16 b. If a failure is detected, the interlock circuit 10 may be partially or completely replaced, thereby further enhancing the reliability of the safety interlock circuit 10.

FIG. 4 is schematic circuit diagram that generally corresponds to FIG. 3, but shows the circuit configuration of the diagnostics unit 34 in additional detail. The same reference numerals are used to designate corresponding components.

As can be taken from FIG. 4, the diagnostics unit 34 comprises a first comparator circuit 36 supplied by the input voltage VIN and connected to the supply line 14 at the position between the first semiconductor transistor element 12 a and the second semiconductor transistor element 12 b. The first comparator circuit 36 provides at its output a voltage VFAIL_0V. The diagnostics unit 34 further comprises a second comparator circuit 38 supplied by the input voltage YIN and connected to the output voltage VOUT of the supply line 14. The output of the second comparator circuit 38 is provided to a third comparator circuit 40, and compared these with the output of the first comparator circuit 36 to yield an output voltage VOUT_MISMATCH.

Table 1 shows the resulting voltages VOUT_TEST, VOUT_SENSE, VOUT_MISMATCH, and VFAIL_0V obtained in the interlock circuit 10 according to FIG. 4 for the example of an input voltage VIN of 32 V, both for the normal operation in which the first and second semiconductor transistor elements 12 a, 12 b function properly and for configurations in which failures occur either in the first or second semiconductor transistor elements 12 a, 12 b. As can be taken from a comparison of the resulting voltage values, the detected voltages in the single fault condition state can be different from the normal state operation, and hence from a comparison the single fault conditions may be reliably inferred. For instance, a fault condition may be inferred if it is detected that the voltage VOUT_MISMATCH differs from 32 V, for instance falls below 25 V. Similarly, a fault condition may be concluded if the voltage VFAIL_0V falls below 32 V, and a threshold for detecting a failure may be set at a voltage lower than 32 V, such as 25 V. In general, threshold voltages may be set for the plurality of detected voltage values VOUT_SENSE, VOUT_MISMATCH and VFAIL_0V to detect failures and distinguish between different single fault condition states.

TABLE 1 Safety Interlock Safety Interlock Mosfets followsVout test State Switch 12a 12b Vout_test Vout_TEST_sense Vout Vout_SENSE Vout_MISMATCH VFAIL_OV Normal State closed OK OK 32 V 16 V 32 V 16 V  32 V 32 V open OK OK 16 V 16 V  0 V 0 V 32 V 32 V Single Fault closed Fail Open OK 0 0  0 V 0 V 0 0 condition State open Fail Open OK open Fail Closed OK 32 V 16 V  0 V 0 V 0 32 V closed Fail Closed OK closed OK Fail Open 32 V 16 V  0 V 0 V 0 32 V open OK Fail Open open OK Fail Closed 0 V (load  0 V  0 V 0 V do not care 0 avoid 16 V closed OK Fail Closed

FIG. 5 is a flow diagram that illustrates an example of an interlock method according to the present disclosure.

At S10, a trigger signal is received. In particular, a trigger signal may be received at a plurality of control units, such as the first and second control units 16 a, 16 b described above with reference to FIGS. 1 to 4. For instance, said trigger signal may be transmitted by the trigger unit 20 and may indicate that one or several of the switch elements 22 a to 22 c have been opened.

At S12, a corresponding semiconductor switching unit may be selectively activated or deactivated. As an example, at S12, by means of each said control unit, a corresponding semiconductor switching unit is selectively activated or deactivated in response to said trigger signal, wherein the semiconductor switching units can be electrically connected in series in a supply line of an electrical appliance.

For example, with reference to the circuit diagrams of FIGS. 1 to 4, the control units 16 a, 16 b may selectively activate or deactivate the corresponding first and second semiconductor transistor elements 12 a, 12 b in response to the trigger signal, wherein the first and second semiconductor transistor elements 12 a, 12 b are electrically connected in series in the supply line 14.

A method with some or all of the features described above can be implemented in hardware. A hardware implementation is particularly reliable and failure-resistant.

However, the disclosure is not so limited, and alternatively, the method may be fully or partially implemented in any combination of hardware and computer executable code. For example, various examples of the present disclosure can be implemented as computer executable code executed by a processor that cause the processor to be configured to perform the functionality of various components described herein. In one particular example, examples of the present disclosure can be implemented by a processor executing computer executable code that includes instructions that cause the processor to perform a method with some or all of the features described above. In one example, such computer executable code can be stored on transitory or non-transitory computer readable media.

In general, an interlock circuit according to the disclosure may comprise a plurality of semiconductor switching units electrically connected in series or electrically connectable in series in a supply line, a plurality of control units electrically connected to said plurality of semiconductor switching units, and a trigger unit electrically connected to said plurality of control units. Each said control unit may activate or deactivate a corresponding semiconductor switching unit in response to a trigger signal received from said trigger unit.

Semiconductor switching units reduce the manufacturing complexity and costs of the interlock circuit. At the same time, the use of a plurality of control units that are electrically connected to a plurality of corresponding semiconductor switching units provides a fully redundant interlock circuit that allows to reliably switch off said electrical appliance in response to said trigger signal even in situations in which one of the semiconductor switching units and/or one of the control units should fail.

Said supply line may be a supply line of an electrical appliance. Said electrical appliance may be any device or machinery for which a safety interlock functionality may be desired, or part of said device or machinery. For example, said electrical appliance may comprise a printing device, or part of a printing device.

Said supply line may be any power or control line of said electrical appliance adapted to switch off or deactivate at least part of said electrical appliance.

Said activation or said deactivation of said semiconductor switching unit may interrupt said supply line of said electrical appliance, thereby deactivating said electrical appliance in response to said trigger signal received from said trigger unit.

Said trigger signal may be any signal signaling an event in response to which an interlock of said electrical appliance is desired. For instance, said trigger unit may comprise a sensor unit, and said sensor unit may be adapted to sense a dangerous operation condition of said electrical appliance that may cause harm or injury to a user, such as an open lid or cover.

Said interlock circuit may comprise a plurality of interlock paths, each interlock path comprising a control unit and a corresponding semiconductor switching unit electrically connected in series. Said interlock paths may be electrically connected in parallel to said trigger unit.

In an example, said semiconductor switching units comprise a semiconductor transistor element, such as a field effect transistor, such as a MOSFET.

Semiconductor transistor elements peiaiiit reliable switching operations, and are available off-the-shelf in a large variety of configurations and at low cost, thereby significantly reducing the manufacturing complexity and costs of the interlock circuit.

In an example, the interlock circuit comprises two semiconductor switching units and two control units, wherein a first control unit is electrically connected to a first semiconductor switching unit, and a second control unit is electrically connected to a second semiconductor switching unit.

An interlock circuit with two semiconductor switching units and two control units provides two independent interlock paths to deactivate said electrical appliance in response to said trigger signal, and thereby provides redundancy against failures.

For example, said first control unit may activate or deactivate said first semiconductor switching unit in response to said trigger signal received from said trigger unit, and said second control unit may activate or deactivate said second semiconductor switching unit in response to said trigger signal received from said trigger unit.

In an example, said plurality of control units may be provided and connected redundantly, so that a first control unit activates or deactivates a corresponding first semiconductor switching unit in response to said trigger signal even if a second control unit fails to activate or deactivate a corresponding second semiconductor switching unit in response to said trigger signal, wherein said second control unit is different from said first control unit and said second semiconductor switching unit is different from said first semiconductor switching unit.

Providing redundancy with two independent control units and two corresponding independent semiconductor switching units allows to reduce the probability of a failure of the interlock system from p to p², wherein p denotes the probability that a failure occurs in a single control unit or semiconductor switching unit.

In an example, said plurality of control units may be spatially separated from one another. In one particular example, the plurality of control units may be spatially separated by at least 2.5 mm.

Similarly, said plurality of semiconductor switching units may be spatially separated from one another. In one particular example, the plurality of semiconductor switching unites can be spatially separated by at least 2.5 mm.

The spatial separation can help ensure that the control units and switching units operate and fail independently, and thereby enhances the failure tolerance of the interlock circuit.

Each said control unit may comprise a resistor element and a semiconductor transistor element electrically connected in series.

The series connection of at least one resistor element and at least one semiconductor transistor element allows for an efficient and reliable control of the corresponding semiconductor switching unit in response to a trigger signal. In particular, the series connection can avoid undesired connections in case of a short/open failure in one element. Moreover, the control unit in accordance with the example can be fabricated reliably and at low cost.

Each said control circuit may comprise a least two independent control paths for activating or deactivating said corresponding semiconductor switching units in response to said trigger signal.

Two independent control paths provide redundancy also in the control circuits, thereby increasing the reliability of the interlock circuit.

Each control path may comprise at least one resistor element and at least one semiconductor transistor element.

In an example, each said control circuit comprises a plurality of resistor elements and a plurality of semiconductor transistor elements, wherein a first resistor element is electrically connected in series with a first semiconductor transistor element, and a second resistor element is electrically connected in series with a second semiconductor transistor element, in parallel with said series connection of said first resistor element and said first semiconductor transistor element.

By means of the parallel connection according to the example, two redundant control paths may be provided in each said control circuit, thereby further enhancing the failure resilience.

In an example, said trigger unit comprises at least one switch element. For example, the trigger unit can include a plurality of switch elements connected in series.

Each said switch element may be placed at a safety-relevant component of said electrical appliance, such as a lid or cover, and may open in response to a safety risk. Said opening of said switch element may interrupt an electrical circuit, and said interruption may constitute or activate a trigger signal in response to which said control unit activates or deactivates said corresponding semiconductor switching unit.

In an example, an interlock circuit further comprises a diagnostics unit electrically connected or electrically connectable to said plurality of semiconductor switching units and/or to said plurality of control units. Said diagnostics unit may test a functionality of said semiconductor switching units or said control units, respectively.

For instance, by means of said diagnostics unit, a user of said electrical appliance may test whether said interlock circuit is still fully operational. In case said diagnostic unit reveals a failure in said interlock circuit, said interlock circuit may be replaced, thereby further enhancing the safety level provided by said interlock system.

In an example, said diagnostics unit may be electrically connected or electrically connectable to said supply line.

An electrical connection of said diagnostics unit to said supply line may allow to test the functionality of said semiconductor switching units connected in series in said supply line, and may likewise allow to test the functionality of said corresponding control units via said respective semiconductor switching units.

In an example, said diagnostics unit comprises a plurality of comparator circuits. Said comparator circuits may allow to compare a voltage value obtained in said interlock circuit in response to an applied test voltage against a predetermined voltage value, thereby allowing to assess said functionality of said interlock circuit.

An interlock method according to the present disclosure may comprise receiving a trigger signal at a plurality of control units, and, by means of each said control unit, selectively activating or deactivating a corresponding semiconductor switching unit in response to said trigger signal, said semiconductor switching units being electrically connected in series in a supply line.

Said supply line may be the supply line of an electrical applicance.

In an example, selectively activating or deactivating said corresponding semiconductor switching unit comprises activating or deactivating a first switching unit by means of a first control unit in response to said trigger signal, and redundantly activating or deactivating a second switching unit electrically connected in series with said first switching unit by means of a second control signal in response to said trigger signal.

In an example, said method further comprises testing a functionality of said control units and/or said semiconductor switching units, wherein testing said functionality may comprise applying a test voltage to said supply line.

Testing said functionality may comprise activating said trigger signal.

In an example, testing said functionality may comprise determining an output voltage. In one example, the output voltage can be obtained in response to applying said test voltage to said supply line, and comparing said output voltage against a predetermined voltage range or voltage value.

The present disclosure further relates to a computer-readable program comprising computer-readable instructions, or to a non-transitory computer-readable medium comprising computer-readable instructions, wherein said instructions, when executed by a processor or computer, cause the processor or computer to be configured to test a functionality of a plurality of semiconductor switching units provided in series in a supply line of an electrical appliance, wherein said test comprises applying a test voltage to said supply line, determining an output voltage, and comparing said output voltage against a predetermined voltage range.

The description of the examples and the figures merely serve to illustrate the disclosure, but should not be understood to imply any limitation. The scope is to be determined solely by means of the appended claims.

Reference Signs

-   10 interlock circuit -   12 a, 12 b first, second semiconductor transistor elements -   14 supply line -   16 a, 16 b first, second control units -   18 trigger line -   20 trigger unit -   22 a, 22 b, 22 c switch elements of trigger unit 20 -   24 a first semiconductor transistor of first control unit 16 a -   24 b first semiconductor transistor of second control unit 16 b -   26 a, 26 a′ first resistor elements of first control unit 16 a -   26 b, 26 b′ first resistor elements of second control unit 16 b -   28 a second semiconductor transistor of first control unit 16 a -   28 b second semiconductor transistor of second control unit 16 b -   30 a, 30 a′ second resistor elements of first control unit 16 a -   30 b , 30 b′ second resistor elements of second control unit 16 b -   32 a first control unit supply line for first control unit 16 a -   32 b second control unit supply line for second control unit 16 b -   34 diagnostics unit -   36 first comparator circuit of diagnostics unit 34 -   38 second comparator circuit of diagnostics unit 34 -   40 third comparator circuit of diagnostics unit 34 

1. An interlock circuit, comprising: a plurality of semiconductor switching units electrically connected or connectable in series in a supply line; a plurality of control units electrically connected to said plurality of semiconductor switching units; and a trigger unit electrically connected to said plurality of control circuits; wherein each said control unit activates or deactivates a corresponding semiconductor switching unit in response to a trigger signal received from said trigger unit.
 2. The interlock circuit according to claim 1, wherein said semiconductor switching units comprise a semiconductor transistor element.
 3. The interlock circuit according to claim 1, wherein said plurality of control units are provided and connected redundantly so that a first control unit in the plurality of control units activates or deactivates a corresponding first semiconductor switching unit in the plurality of semiconductor switching units in response to said trigger signal even if a second control unit in the plurality of control units fails to activate or deactivate a corresponding second semiconductor switching unit in the plurality of semiconductor switching units in response to said trigger signal, wherein said second control unit is different from said first control unit and said second semiconductor switching unit is different from said first semiconductor switching unit.
 4. The interlock circuit according to claim 1, wherein each said control unit comprises a resistor element and a semiconductor transistor element electrically connected in series.
 5. The interlock circuit according to claim 1, wherein each control circuit in the plurality of control units comprises a plurality of resistor elements and a plurality of semiconductor transistor elements, wherein a first resistor element is electrically connected in series with a first semiconductor transistor element, and a second resistor element is electrically connected in series with a second semiconductor transistor element, in parallel with said series connection of said first resistor element and said first semiconductor transistor element.
 6. The interlock circuit according to claim 1, wherein said trigger unit comprises a plurality of switch elements connected in series.
 7. The interlock circuit according to claim 1, further comprising a diagnostics unit electrically connected or electrically connectable to said plurality of semiconductor switching units or to said plurality of control units to test a functionality of said semiconductor switching units or said control units.
 8. The interlock circuit according to claim 7, wherein said diagnostics unit is electrically connected or electrically connectable to said supply line.
 9. The interlock circuit according to claim 7, wherein said diagnostics unit comprises a plurality of comparator circuits.
 10. The interlock circuit according to claim 1, wherein said supply line is disposed in a printing device or part of a printing device.
 11. An interlock method, comprising: receiving a trigger signal at a plurality of control units; and by means of each control unit in the plurality of control units, selectively activating or deactivating a corresponding semiconductor switching unit in response to said trigger signal, said semiconductor switching units being electrically connected in series in a supply line.
 12. The method according to claim 11, wherein selectively activating or deactivating said corresponding semiconductor switching unit comprises activating or deactivating a first switching unit in the plurality of switching units by means of a first control unit in the plurality of control units in response to said trigger signal, and redundantly activating or deactivating a second switching unit in the plurality of switching units electrically connected in series with said first switching unit by means of a second control signal in the plurality of control units in response to said trigger signal.
 13. The method according to claim 11, further comprising testing a functionality of said plurality of control units or said plurality of semiconductor switching units, wherein testing said functionality comprises applying a test voltage to said supply line.
 14. The method according to claim 13, wherein testing said functionality comprises: determining an output voltage of said interlock circuit; and comparing said output voltage against a predetermined voltage range or voltage value.
 15. A non-transitory computer-readable medium comprising computer-readable instructions, wherein said instructions, when executed by a processor cause the processor to be configured to: test a functionality of a plurality of semiconductor switching units provided in series in a supply line; wherein testing said functionality comprises applying a test voltage to said supply line; determine an output voltage of said supply line; and compare said output voltage against a predetermined voltage range or voltage value. 